Dual antenna system for transponder beacon devices



E. HART ET AL 3,375,516

6 Sheets-Sheet l March 26, 1968 DUAL ANTENNA SYSTEM FOR TRANSPONDER BEACON DEVICES Filed May 26, 1967 s 22.5w m m m y w N. V M. O N T 6 W I R w A .A 0 S625 glim ei ojfitimimj H mm 3 *2 E. E 7 89$ 55% S l M fin ow M A R 2 m M 6 C wmumkam 10.5523 5 5". mohizoi mmwmnanw 502 UN EN mN L 6 Sheets-Sheet f 8 235m R m 0 m r m N M E m R w 5625. moz 5eojfit zmz mj M 0 3 8 2v iQdQT $2523 ww 5n T EQ8mQT 8051 528% it 4 F 5 mm 2m 3 mm on D E l L L m. A R m m 6 C G. E. HART ET AL mmwmmmaw March 26, 1968 DUAL ANTENNA SYSTEM FOR TRANSPONDER BEACON DEVICES Filed May 26, 1967 muoQE.

w wI

March 26, 1968 G. E. HART ET AL DUAL ANTENNA SYSTEM FOR TRANSPONDEH BEACON DEVICES 6 Sheets-Sheet 5 Filed May 26, 1967 KEEN-3E tm mwwwm 955; EEHE Em A J 8 2 TWMQM mozmof $Ezmz E L 8 mm EN EOhFOm mm mmummnSM On mwmmmmnm moh INVENTORS GERALD E. HART CHARLES E. QU/G'LEY ATTORNEYS March 26, 1968 I G. E. HART ET AL 6 Sheets-Sheet 4 Filed May 26, 1967 INVENTORS ATTORNEYS Y 3 3 M T 6 T3330: 15:22:; R w m o mwmfi m J E J SQNSI fizwouw M N on S D E k L L A R R A a w. E8 5 588a mo Y 3 3 3 238m B 2552 296m lln E 88 6 j zmoz $2523 M32 525128 1055 M505 Elm/ 1 U u Fl II 52% 5:5 DE

1 8 2 5:33 A one? 5581? 2 :HF. 8 a 2 F 2 mmwmmmam March 26, 1968 G. E. HART ET AL DUAL ANTENNA SYSTEM FOR TRANSPONDER BEACON DEVICES 6 Sheets-Sheet 5 Filed May 26, 1967 muooomo imobjso mmummmam moh mmzuw um sa c4128 Nv SQ TTE GSE ON mmummmnm m 2923 m A? E Y N. W M vT 6 i M w n H o Y. E. E. s w w A R R A u M zsozw m SzoEmE 5 3218. \556 -85 5% n 9 ATTORNEYS 6 Sheets-Sheet 6 G. E. HART ET DUAL ANTENNA SYSTEM FOR TRANSPONDER BEACON DEVICES March 26, 1968 Filed May 26, 1967 m O zotbm m E N. W M .3 o T 6 I R U r l mohfiaoogtimzh ill! 11 M O E5 wwwEw 5 $805 $880 mo A 8?. 528mm fi F 8 N" on E 3 g N@ L L m M zaozm 3 E H S20E25 5 2252 6 0 E8 10:5 U8 5% mo 8 $2550 U32 moEmzoo mv 52% 5:2 u m 2396 m 8 82958 5 22%: \zotam E05 5% H Sei E581 a Q A. i mmummmzm N m s I Y :6- ATTORNEYS strength.

United States Patent O Navy Filed May 26, 1967, Ser. No. 643,023 10 Claims. (Cl. 343--6.8)

ABSTRACT OF THE DISCLOSURE Transponder system having dual antennas. Different switching arrangements whereby transponder response is through antenna selected on basis of intercepted signal The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION Field the invention Transponder systems are widely used in civil and military airtrafiic control systems to determine the identity and/or altitude of aircraft and in military IFF (Identification Friend or Foe) systems to establish the military identity of airplanes. In these systems an interrogation station, which may either be located on the ground or on another airplane, transmits a coded interrogation signal, often by radar. Transponders, which are carried on the airplanes interrogated, are triggered by the coded interrogation signal to produce and transmit a response (or reply) signal whichis also coded and which, when received by the interrogation station, furnishes the desired information to the operator of the interrogation station. The contemplated environment of this invention is in the responding portions of transponders systems which are carried on airplanes.

Description of the prior art Previous transponder systems which used a single transponder antenna were not completely satisfactory in that the single antenna did not provide adequate coverage. Although located on the bottom of the airplane, the single transponder antenna was frequently shadowed from the interrogation ground station during turns and banks and particularly when interrogation was from another airplane at a higher altitude.

To overcome the shadowing disadvantage of single transponder antenna systems, some prior transponder systems have also utilized dual antennas, i.e. an antenna on both the top and bottom of the responding airplane. If operated in parallel, that is with both antennas always connected to the transponder, the dual antenna systems are unsatisfactory in that the radiation pattern is not uniform because df cancellation nulls. Programmed switching, whereby the antennas are alternately and individually connected to the transponder, has also been used in prior art dual antenna systems, but is not entirely satisfactory because of 'the signal loss whenever an antenna which is shaded from the interrogation station is connected to the transponder.

Summary of the invention The present invention avoids the shortcomings of prior transponder antenna systems by providing dual antennas which are individually and selectively connected to the transponder transmitter, the connection being selected 3,375,516 Patented Mar 2 6, 1968 primarily on the basis of which antenna most strongly receives the interrogation signal. To accomplish the selective connection, the invention includes a plurality of different signal comparing and switching arrangements, each of which may be most preferable in certain circumstances.

It is, therefore, an object of the invention to provide an improved transponder system;

Another object is to provide an improved transponder system which includes dual antennas.

Still another object is the provision of an improved transponder system which includes dual antennas that are individually and selectively connected to the transponder transmitter in a manner related to the strength of the interrogation signal intecepted by each antenna.

A still further object is to provide an improved transponder system which includes dual antennas which are individually and selectively connected to the transponder transmitter, the connection being selected primarily on the basis of which antenna most strongly receives the interrogation signal.

Brief description of the drawings These and other objects and features of the invention will become apparent to those skilled in the art as the disclosure is made in the following description of preferred embodiments of the invention which are illustrated in the accompanying drawings in which:

FIG. 1 illustrates a first embodiment of the invention;

FIG. 2 illustrates a second embodiment of the invention;

FIG. 3 shows a third embodiment of theinvention;

FIG. 4 is a block diagram of a fourth embodiment of the invention;

FIG. 5 illustrates a fifth embodiment of the invention; and a FIG. 6 shows a sixth embodiment of the invention.

characters designate like or corresponding parts through-v out the several views, there is shown in FIG. 1 (which illustrates a first preferred embodiment of the invention) a top antenna 10 and a bottom antenna 12 respectively connected to conventional transponders 1 4 and 16 which, per Se, do not form part of the present invention and which are illustrated only to the extent necessary for describing the present invention. Transponder 14 includes a receiver 18, video circuits 20 and decoder 22 connected in series for receiving an interrogation signal and an encoder 24, modulator 26 and transmitter 28 connected in series for transmitting a reply signal. Transponder 16 similarly includes receiver 30, video circuits 32, decoder 34, encoder 36, modulator 38 and transmitter 40.

Comparator-42 is connected to receive the video outut signals from top transponder 14 and bottom transponder 16 and in turn produces a signal the polarity of which is indicative both of which of the video signals is the larger and of which of the antennas 10 or 12 more strongly intercept the interrogation signal. If the top antenna 10 intercepts the stronger interrogation signal, the polarity of the signal from comparator 42 is such that the gate 44 passes the signal from decoder 22 to trigger pulse generator 46 which in turn is connected to trigger encoder 24 to initiate a replay signal through If the bottom antenna intercepts the stronger interrogation signal, the comparator 42 will not be effective to open the gate 44. Rather, the signal from decoder 34 will, after a slight delay in component 52, pass through gate 50 and energize the pulse generator 54 which is in turn connected to trigger encoder 36 to initiate a reply signal through modulator 38, transmitter 40 and bottom antenna 12. The signal from pulse generator 54 is also connected to suppress decoder 22, thereby keeping the top transponder 14 from transmitting a replay signal through the antenna 10.

The operation of the embodiment of the invention illustrated in FIG. 1 is by now apparent. If, as is usual, the bottom antenna 12 intercepts the stronger interrogation signal, the comparator 42 does not participate and allows both the usual energization of encoder 36 (by decoder 34, gate 50 and pulse generator 54) to transmit a reply signal via bottom antenna 12 and the usual suppression of the top transponder 14 (by pulse generator 54). In the unusual event when the top antenna intercepts the stronger signal, the comparator 42 acts to open gate 44 to allow decoder 22 both to energize encoder 24 (by means of pulse generator 46) to transmit a reply signal via top antenna 10 and to suppress the bottom transponder 16.

The signal from bottom decoder 34' is connected through delay 52, which typically can be of 0.5 microsecond, to ensure that inhibit action of pulse generator 46, when the top antenna 10 intercepts the stronger signal, precedes the trigger from decoder 34 occurring at gate 50. The width of the output pulse from pulse generator 46, which typically can be 1.5 microsecond, is sufficient to inhibit the trigger from delay 52 at gate 50. In this manner a reply signal is transmitted via only one antenna, Whose selection is primarily on the basis of which antenna most strongly receives the interrogation signal.

It will be recognized that the embodiment of the invention illustrated in FIG. 1 is advantageous in that it utilizes conventional transponders which do not require modification. However, the use of two transponder units necessarily entails large space and weight requirements which may be a major disadvantage in certain installations. A further and not quite so obvious advantage implicit in the embodiment of FIG. 1 is that the circuits which are part of the present invention, in order not to interfere with normal operation of the transponder, must function rapidly in response to each individual interrogation signal.

The performance of the embodiment of FIG. 1 also may not be completely satisfactory due to possible limited dynamic range of receivers 18 and 30 and video circuits and 32, which may cause possible loss of close-in replies. Certain close-in maneuvers may result in the transponder reply being transmitted from a shadowed antenna. The shadowing may be sufficiently great, coupled with the gain time control of the interrogation station receiver (to avoid side lobe response problems), that the signal will be of too low strength to be detected by the receiver which is set at very low gain at short ranges. Further, and of even more serious consequence, the embodiment of FIG. 1 is not entirely satisfactory inasmuch as time jitter may occur during reply path transition, caused by additional time delay 52 in the bottom transponder trigger circuit, with the likelihood that a defruiter circuit in the interrogation station receiver will throw out the reply signal or that the automatic detection system will report two targets.

Although the embodiment of FIG. 1 is considered to be a substantial improvement over prior art transponder antenna systems, because of the above described limitations to the performance of the embodiment of FIG. 1, it is often desirable to use embodiments of the invention which are more sophisticated and which avoid the described shortcomings of the embodiment of FIG. 1.

Referring now to FIG. 2, there is shown a second embodiment of the invention wherein the reply signal is transmitted by the bottom antenna 12 unless the interrogation signal intercepted by antenna 12 is so weak as to fail to trigger the reply circuitry. The embodiment of FIG. 2 is similar to that of FIG. 1 in that complete and conventional transponders 14 and 16 are connected to top and bottom antennas 10 and 12. As previously described the top transponder 14 includes receiver 18, video circuits 20, decoder 22, encoder 24, modulator 26 and transmitter 28 and the bottom transponder 16 includes receiver 30, video circuits 32, decoder 34, encoder 36, modulator 38 and transmitter 40, all connected to operate as in the embodiment of FIG. 1.

The bottom decoder 34 is connected to pulse generator 54 which is in turn connected through delay 56 to both energize encoder 36 to initiate a reply signal through modulator 38, transmitter 40 and bottom antenna 12 and to suppress decoder 22 of the top transponder 14.

The decoder 22 of the top transponder 14 is connected through delay 58 to gate 60. Gate 60 is connected to be inhibited by the signal from pulse generator 54 and in the absence of such a signal will pass the delayed signal from decoder22 to pulse generator 46 which is in turn connected to both energize encoder 24 to initiate a reply signal through modulator 26, transmitter 28 and top antenna 10 and to suppress decoder 34 of the bottom transponder 16. The delay 56, which typically is of 0.30 microsecond, is for the purpose of balancing the effect of delay 58 so as to reduce transponder jitter during periods when replies are shifting from the top transponder 14 to the bottom transponder 16 and vice versa.

The operation of the embodiment of FIG. 2 is such that the reply signal will be transmitted by bottom antenna 12 if that antenna intercepts the interrogation signal with suificient strength to energize pulse generator 54 and without regard to the strength of the interrogation signal intercepted by top antenna 10. The top transponder system is disabled, whenever the bottom system is operative, by the signal from pulse generator 54 to the gate 60. In other respects the embodiment of FIG. 2 is similar to that of FIG. 1.

The embodiment of FIG. 2 is considered to be operationally superior to that of FIG. 1 with respect to jitter.

However, as with FIG. 1, the embodiment of FIG. 2 has the weight and space disadvantages inherent in the use of the two complete transponder units 14 and 16. Also, as with FIG. 1, the embodiment of FIG. 2 is still subject to possible loss of close-in replies, since transition to the bottom antenna 12 in this latter system does not depend on signal strength at the top antenna. Certain close-in maneuvers may result in the transponder reply being transmitted by the bottom antenna while it is shadowed, with consequent signal strength insufficient to be detected by the interrogation station receiver, which is set at very low gain at short ranges.

The embodiment of FIG. 3 overcomes (to some extent) the weight and space disadvantages of the previously described embodiments by using the same decoder and encoder units for both the top and bottom antenna systems together with a comparator controlled gating system which directs the reply signal to the antenna receiving the stronger interrogation signal.

Referring now to FIG. 3, the antennas 10 and 12, the receivers 18 and 30, the video circuits 20 and 32, the transmitters 28 and 40 and the modulators 26 and 38, as well as the comparator 42, gate 44 and pulse generator 46 are connected and function similarly to the operation and cooperation of these components in the previously described embodiment illustrated in FIG. 1.

The output of video circuits 20 and 32, in addition to being connected to comparator 42, are also connected to an OR gate 62 which is in turn connectedto decoder 64. The decoder 64 is connected to both energize encoder 66 and to gate 44 which is enabled by the signal from comparator 42 whenever the interrogation signal intercepted by top antenna is stronger than the interrogation signal intercepted by the bottom antenna 12. The signal from pulse generator 46 (which occurs only when the interrogation signal intercepted by the top antenna 10 is the stronger) is connected to both gated amplifier 68 and (through inverter 48) to gated amplifier 70. The gated amplifier 68 is normally biased off, that is, not to pass a signal, and is turned on, i.e., to pass the encoder signal to modulator 26, by the signal from pulse generator 46. The gated ampilfier 70 is normally biased on, that is, to pass a signal from encoder 66 to modulator 28, and is turned off, i.e., to not pass a signal, by the inverted signal from pulse generator 46.

It will be apparent that the operation of the embodiment of FIG. 3 is such that for the usual circumstance, that is, when the bottom antenna 12 intercepts the interrogation signal most strongly, the reply signal produced by encoder 66 will be passed through amplifier 70, modulator 38 and transmitter 40 to be radiated by bottom antenna 12 and that the encoder signal will not reach the top modulator 26 because this signal will not be passed by the ott biased amplifier 68. In the event the top antenna 10 more strongly intercepts the interrogation signal, the comparator 42 will energize the pulse generator 46 to bias the amplifiers 68 and 70 so that the signal from the encoder 66 will not reach the bottom modulator 38 but will reach top modulator 26 and be transmitted by top antenna 10.

It is, of course, fairly obvious that the embodiment of FIG. 3 possesses significant advantages in space and weight requirements (when compared to the embodiments of FIGS. 1 and 2) because the embodiment of FIG. 3 requires only one decoder and one encoder. A further and not quite so obvious advantage possessed by the embodiment of FIG. 3 is that the reply signal is less garbled by jitter than in the previously described embodiments which use both top and bottom decoders and encoders. To accomplish this, the same decoder and encoder function to produce the reply signal without regard to which antenna intercepts the stronger interrogation signal.

A further reduction in the required components is provided by the embodiment illustrated in FIG. 4 wherein single modulator and transmitter units (as well as decoder and encoder units) are used to selectively radiate signals from the top and bottom antennas. To accomplish this, the embodiment of FIG. 4 utilizes switching in the RF leads to the antennas rather than video switching as in the previously described embodiments.

Referring now to FIG. 4, the bottom antenna 12 is connected to receiver 30 and transmitter 40 through a switch 72, preferably a SPDT diode type, that is normally closed to the bottom antenna. Top antenna 10 is also coupled to the normally open connection of switch 72 through T coupler 74 and tuned-line 76. The other arm of coupler 74 is connected to receiver 18 by tunedline 78. The lines 76 and 78 are tuned, as is conventional practice, so that the one not in use during either transmission or reception will reflect an open circuit at the coupler 74.

Receiver 18 is connected to comparator 42 and OR gate 62 through the video circuits 20. Bottom receiver 30 is similarly connected through the video circuits 32. As described previously in connection with the embodiment of FIG. 3, the OR gate 62, decoder 64 and encoder 66 function to produce a reply signal without regard to which antenna intercepts the stronger interrogation signal whereas the comparator 42, gate 44 (which is connected to the output of decoder 64) and pulse generator 46 function to produce a signal only if the interrogation signal intercepted by top antenna 10 is stronger than the interrogation signal intercepted by bottom antenna 12. The output of pulse generator 46 is connected to the driving circuitry 80 of switch 72 and functions to change the switch condition from the closed bottom antenna condition illustrated to the condition wherein the tuned line 76 is connected to the transmitter 40. Transmitter 40 is in turn connected through modulator 38 to the output of encoder 66 which is also connected to the Suppress terminal of receiver 18 and to the Suppress terminal of receiver 30.

The operation of the embodiment of the invention illustrated in FIG. 4 is by now apparent. If the bottom antenna 12 intercepts the stronger interrogation signal, the comparator 42 does not cause the pulse generator 46 to be triggered and the switch 72 remains in the normal (closed to bottom antenna) condition wherein the reply signal produced by encoder 66 is connected through modulator 38, transmitter 40 and normally conditioned switch 72 to be radiated by bottom antenna 12. When the top antenna 10 intercepts the stronger interrogation signal, the comparator 42 energizes the pulse generator 46 to produce a signal which'causes the switch 72 to connect the top antenna 10 (through coupler 74) to the transmitter 40. When the switch 72 is so conditioned, the reply signal producedby encoder 66 is connected through modulator 38, transmitter 40 and abnormally conditioned switch 72 to be radiated by top antenna 10. A small but sufiicient time delay exists in encoder 66 so that switch 72 connects the top antenna 10 to the transmitter 40 prior to commencement of the reply signal.

The embodiment of FIG. 4 is obviously advantageous compared to previously described embodiments of the invention in that only one transmitter unit and only one modulator unit is required. Further, this embodiment performs relatively well as to jitter and to loss of close-in replies. However, the tuned lines 76 and 78 can give rise to operational problems by becoming detuned in service. The embodiment of FIG. 5 overcomes this shortcoming by using two RF switches in order to eliminate the need for the tuned RF lines.

The embodiment illustrated in FIG. 5 is identical to that of FIG. 4 except that top antenna 10 is connected (in the normal condition) by switch 82 to receiver 18. Switch 82 is similar to the previously described switch 72, and like that switch is connected to be controlled by the driver 80. The normally open terminals of switches 72 and 82 are connected together as shown.

In operation the embodiment of FIG. 5 is also similar to the embodiment of FIG. 4. When the bottom antenna 12 intercepts the stronger interrogation signal, the comparator 42 does not cause the pulse generator 46 to be triggered and the switches 72 and 82 remain in the conditions illustrated and the reply signal produced by encoder .66 is connected through modulator 38, transmitter 40 and normally conditioned switch 72 to be radiated by bottom antenna 12. When the top antenna 10 intercepts the stronger interrogation signal, the comparator 42 energizes the pulse generator 46 to produce a signal which causes the switches 72 and 82 to connect the top antenna 10 to the transmitter 40. The reply signal produced by encoder 66 is connected through modulator 38, transmitter 40, abnormally conditioned switches 72 and 82 to be radiated by top antenna 10. It will be recognized, of course, that a working design of the embodiment of FIG. 5 must include a provision, such as a short duration lock-in feature in the driver circuit 80, whereby the switches, once thrown to their abnormal conditions, will remain in their abnormal conditions until the encoder reply signal has been transmitted. Without this feature, the disconnection of the top antenna from receiver 18 will cause the switches 72 and 82 to be returned to their normal conditions.

Whereas the embodiments of FIGS. 1, 2, 3, 4 and 5 deal with transponder system equipped only for operation on a single interrogation mode, the basic techniques described herein can obviously be employed with transponder systems equipped for reply in response to more than one interrogation mode.

The embodiment illustrated in FIG. 6 is identical with that of FIG. 5 except that OR gate is employed to 7 accept triggers from any one of various output leads from decoder 92 corresponding to the interrogation mode being received at any moment. However, the embodiment of FIG. 6 is similar to that of FIG. in that the combined outputs of decoder 92 appearing at the output of OR gate 90 are allowed to trigger pulse generator 46 through gate 44, when properly energized by comparator 42. Decoder 92, of course, also energizes encoder 94'. Inall other respects, the embodiment of FIG. 6 is similar to that of FIGURE 5.

Readers desiring a more comprehensive disclosure of certain aspects of the invention, such as details of circuitry, component values, tube identification, potential levels, etc. should consult NRL Report No. 6401 of the Naval Research Laboratory, Washington, DC. dated Oct. 18, 1966 and titled The Hartlobe Airborne Dual Antenna System for IFF.

It is by now apparent that there has been disclosed a plurality of embodiments of the invention which provides an improved transponder system that includes dual antennas that are individually and selectively connected to the transponder transmitter in a manner related to the strength of the interrogation signal intercepted by each antenna. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A transponder system comprising: dual antenna means including first and second antennas for intercepting coded-interrogation signals;

transponder means connected to said dual antenna means for receiving said intercepted coded interrogation signals and for producing a coded reply signal and switching means connected to said transponder means for causing said coded reply signal to be radiated fromthe one of said first and second antennas which is selected on the basis of the strengths of'the coded interrogation signals intercepted by said first and second antennas.

2". A transponder system as set forth in claim 1 wherein said coded reply signal is normally radiated by said first antenna and said switching means includes comparator means for comparing the strengths of the coded interrogatory' signals intercepted by said first and second antennas and causes said second antenna to radiate said coded reply signal only when said second antenna intercepts the coded interrogatory signal more strongly than said first antenna.

3. A transponder system as set forth in claim 2 wherein said first and second antennas are each connected to series circuits which each include a receiver, video circuits, a decoder, agate, a pulse generator, an encoder, a modulator and a transmitter and said comparator means produces a signal which enables thegate in the series circuit connected to said second antenna and inhibits the gate in the series circuit connected to said first antenna, said signal being produced by said comparator means when said second antenna intercepts the coded interrogatory signal more strongly than said first antenna.

' 4. A transponder system as set forth in claim 2 wherein said trans-ponder means includes a decoder and an encoder and said first antenna is connected to a series circuit which includes a first receiver, first video circuits, said decoder, said encoder, a first gated amplifier, a first modulator and a first transmitter and said second antenna is connected to a series circuit which includes a second receiver, second video circuits, said decoder, said encoder, a second gated amplifier, a second modulator and a second transmitter and said comparator means produces a signal which reverses the bias of said first and second gated amplifiers and thereby enables said second gated amplifier and inhibits said first gated amplifier, said signal being produced by said comparator means when said second antenna intercepts the coded interrogatory signal more strongly than said first antenna.

5. A transponder system as set forth in claim 2 wherein said transponder means are connected to said dual antenna means by a SPDT diode switch.

6. A transponder system as set forth in claim 5 wherein said transponder means includes a transmitter which is normally connected to said first antenna by said SPDT diode switch and said comparator means produces a signal which causes said SPDT diode switch to connect said transmitter to said second antenna, said signal being produced by said comparator means when said second antenna intercepts the coded interrogatory signal more strongly than said first antenna.

7. A transponder system as set forth in claim 2 wherein said transponder means are connected to said dual antenna means by first and second SPDT diode switches.

8. A transponder system as set forth in claim 7 wherein said transponder means includes a transmitter which is normally connected to said first antenna by said first SPDT diode switch and said comparator means produces a signal which causes said transmitter to be connected to said second antenna by a series connection including both said first and said second SPDT diode switches, said signal being produced by said comparator means when said second antenna intercepts the coded interrogator'y signal more strongly than said first antenna.

9. A transponder system as set forth in claim 8 Wherein said transponder means will selectively function in a plurality of modes according to the mode of said coded interrogatory' signal.

10. A transponder system as set forth in claim 1 wherein said first antenna is connected to a series circuit which includes a first receiver, first video circuits, a first decoder, a first pulse generator, a first encoder, a first modulator and a first transmitter and said second antenna is connected to a series circuit which includes a second receiver, second video circuits, a second decoder, a gate, a second pulse generator, a second encoder, a second modulator and a second transmitter and said first oscillator output is connected to said gate whereby said gate is inhibited Whenever said first oscillator produces an output signal.

References Cited UNITED STATES PATENTS 5/1962 Grifiith 325-304 X 6/ 1963 Silberstein. 

